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α-Fe2O3 nanotube arrays composite for enhanced photocatalytic activity
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Short communication
Construction of novel Z-scheme Cu2O/graphene/α-Fe2O3 nanotube arrays composite for enhanced photocatalytic activity ⁎
Fei Li , Bo Dong School of Mechanical Engineering, Jiangxi University of Technology, Nanchang 330098, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Z-scheme Cu2O/graphene/α-Fe2O3 Composites Photocatalytic degradation Methylene blue
Novel Z-scheme Cu2O/graphene/α-Fe2O3 nanotube arrays (Cu2O/G/FNA) composite was synthesized by facile electrochemical processes for the first time. Compared to single- (1C) or two-component (2C) system, threecomponent (3C) Z-scheme composite material exhibited the efficient photogenerated electron-hole separation as well as the sufficient redox potential via directly quenching between the weak oxidative holes and reductive electrons after Z-scheme charge transfer. The photoelectrochemical measurements and photocatalytic degradation of methylene blue by Cu2O/G/FNA under visible-light irradiation demonstrated the enhanced photoactivity and photostability compared to 1C, 2C, and 3C non-Z-scheme system. The Z-scheme photocatalytic mechanism was confirmed based on the relative characterizations and radical trapping experiments over Cu2O/G/FNA and G/Cu2O/FNA composites, also indicating that •O2− radicals was the prime active species over Cu2O/G/FNA. In addition, G-sheet layer as an efficient electron mediator plays a key role in the construction of Z-scheme 3C composite photocatalyst.
1. Introduction Photocatalysis as an advanced oxidative process using semiconductors such as TiO2 has attracted much attention for wastewater treatment [1–5]. However, due to the large bandgap of TiO2 (~ 3.2 eV) its practical photocatalytic applications are limited at solar light illumination [1]. Comparison with TiO2, hematite (α-Fe2O3, ntype semiconductor) is believed to be a promising candidate due to its suitable bandgap of ~ 2.0 eV for efficient visible-light harvesting, low cost and environmentally friendly characteristics [6]. However, the photocatalytic performance of α-Fe2O3 is still hinted by bulk and surface electron-hole recombination [7]. In order to inhibit this electron-hole recombination, one-dimension α-Fe2O3 nanotube arrays (FNA) was employed as its large specific surface area and short holes transport route [8]. Although this, the problem of surface recombination of photogenerated electron-hole pairs remains. To thoroughly overcome this drawback, two-component (2C) semiconductors heterojunction composite has been extensively studied. Among these alternative semiconductor materials, Cu2O as an attractive p-type semiconductor (2.0–2.2 eV) can be conjugated with hematite to enhance the separation of photogenerated carriers because of the Type II charge transport between their matched energy band positions [7,9,10]. Nevertheless, this traditional charge transfer route between heterojunctions leads to the weak redox potential of photogenerated electrons
⁎
and holes [11]. Recently, Z-scheme photocatalyst composite has been developed which not only retains the efficient photogenerated charge separation but achieves its excellent redox ability [1,11,12]. Although Z-scheme α-Fe2O3/Cu2O heterostructure has been reported so far [7], to our knowledge, the research on Z-scheme FNA-based heterojunction materials with enhanced photocatalytic performance is still limited. On the basis above, we adopted graphene (G) as a good electron mediator to successfully fabricate three-component (3C) Z-scheme Cu2O/G/FNA composite for the first time. Compared to single-component (1C) and 2C systems, it showed the obviously improved photocatalytic activity and photostability for the degradation of methylene blue (MB) under visible-light irradiation (λ ≥ 400 nm). The Z-scheme charge transport route and photocatalytic mechanism were also confirmed on the basis of the relative characterizations, photoelectrochemical (PEC) measurements and radical trapping experiments over pure FNA, 1C, 2C and 3C FNA-based composite materials. 2. Experiments and characterizations The Z-scheme Cu2O/G/FNA composite was prepared by facile and successive electrochemical processes. The detail synthesis procedures are available in the Supplementary information. The morphologies of the samples were observed on JEOL7600F field emission electron microscope (FE-SEM). X-ray diffraction (XRD)
Corresponding author. E-mail address: [email protected] (F. Li).
http://dx.doi.org/10.1016/j.ceramint.2017.08.021 Received 20 May 2017; Received in revised form 18 July 2017; Accepted 2 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, F., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.021
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phenomenon has also been observed in the Cu2O/G/FNA sample (Fig. 1(d)), which reveals the introduction of G has no influence on the morphology of Cu2O nanoparticles and FNA substrate. In addition, for G/Cu2O/FNA, Fig. S2(d) shows that Cu2O nanoparticles are only located on the mouth of FNA, and covered by a few of G layers, which is in accord with non-Z-scheme architecture illustrated in the Fig. S1. Thus, from the results of FE-SEM, it is obviously demonstrated that the 3C layer-structure composites have been successfully fabricated, which is also verified on the basis of XRD, Raman spectra and XPS. The UV–vis DRS show that the main absorption range of bare FNA extends into the visible region (Fig. 2(a)). After coupling with G or Cu2O, the absorption intensity of both the G/FNA and Cu2O/FNA samples decreases to some extent, resulting from the opacity of G [14] and the wider band gap of Cu2O relative to α-Fe2O3 [7]. Although this, Cu2O/G/FNA sample shows the strong visible-light absorption, indicating the practical feasibility as efficient visible-light photocatalyst. However, the 3 C composite G/Cu2O/FNA with the identical components exhibits the apparently poor visible-light absorption due to the block effect of the G layers on the top surface. According to Tauc equation, the band gap energies were calculated using the following Equation [10]:
pattern was performed on D8 Advance X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectra (XPS) and Raman spectra were recorded on an XSAM-800 X-ray Photoelectron Spectroscopy and Spex 403 Raman spectrometer. Photoluminescence (PL) and UV–vis diffuse reflectance spectra (DRS) were taken at room temperature with a fluorospectrophotometer with an excitation at 350 nm light, and a Cary 5000 spectrometer. Photoelectrochemical measurements were performed on an electrochemical workstation in a three-electrode 0.5 M Na2SO4 solution system using the as-prepared sample foils as the working electrodes (cut into 1 cm × 5 cm in size) with a light area of 0.5 cm × 0.5 cm = 0.25 cm2, a Pt plate as the counter electrode, and a Ag/AgCl (saturated in KCl) as the reference electrode. The light source was a xenon lamp irradiation through a filter (λ ≥ 400 nm) with an intensity of 100 mW/cm2. The transient photocurrent curves were measured at + 0.6 V vs. Ag/AgCl under visible-light illumination with on/off light cycles of 50 s intervals. Electrochemical impedance spectroscopy (EIS) tests were conducted at open-circuit voltage under the identical irradiation with frequency in the range of 1–106 Hz and an amplitude of 5 mV. In the photodegradation of MB, the obtained samples (2 × 2 cm2) were placed into a 50 mL MB solution (20 mg/L, pH = 7) under continuous stirring. After 30 min adsorption-desorption equilibrium, the degradation efficiency of MB under visible-light irradiation (λ ≥ 400 nm) were recorded in 60 min by the absorption intensity of the solution at 664 nm in the measured UV–vis spectra. The photocatalytic degradation efficiency of MB were evaluated by the value of C/ C0, where C0 is the absorbance of original MB solution, C is the absorbance of the dye solution after light irradiation. Radical trapping experiments over Cu2O/FNA, Cu2O/G/FNA and G/Cu2O/FNA sample under identical conditions were separately performed by adding 1 mM ammonium oxalate (AO), 1 mM p-benzoquinone (BQ) or 1 mM tert-butyl alcohol (TBA) into the MB solution before the photocatalytic tests to trap holes, •O2− or •OH, respectively.
αhν = A(hν − Eg )η
(1)
where α, h, ν, A and Eg are η absorption coefficient, plank's constant, frequency of light, proportionality constant and band gap energy, respectively. η is a variable which depends on the nature of optical transition (for direct band transition η = 0.5 and for indirect band transition η = 2). Hence, Tauc plots of all samples were shown in the Fig. 2(b) and the calculated Eg was listed in the Table 1. The calculated Eg value of FNA is 2.08 eV, agreeing well with the reported values of α– Fe2O3 [6]. The Eg values of the composites are found to be almost unchanged. This indicates the introduction of G and Cu2O have no remarkable effect on the band gap energy of composites. What's more, Fig. 3 shows the lowest PL intensity of the Cu2O/G/ FNA composite, suggesting the lowest recombination rate of electronhole pairs and longest lifetime of photogenerated carriers. Especially, compared with G/Cu2O/FNA sample, the remarkably lower PL intensity of Cu2O/G/FNA has been exhibited, testifying that Z-scheme charge transport would be in favor of the effective separation of electrons and holes [7,12]. To detailedly characterize the photoresponse of different samples, photoelectrochemical (PEC) measurements were performed in a threeelectrode system. Transient photocurrent responses are recorded over several light on-off illumination cycles to investigate the separation of photogenerated electron-hole pairs, as shown in Fig. 4(a). From the curves of photocurrent over all samples, it is showed that the PEC response of Cu2O/G/FNA is remarkably better than that of other samples, indicating the increasing charge carriers generation and enhanced separation of electron-hole pairs in 3C Z-scheme system in comparison with 1C, 2C and 3C non-Z-scheme system. Additionally, EIS measurement can manifest the conductivity property of the materials under the irradiation. As can be seen in Fig. 4(b), the diameter of semicircle at high frequency in a Nyquist plot, equal to the charge transfer resistance, over Cu2O/G/FNA sample is much smaller than those of pure FNA and G/Cu2O/FNA, implying its enhanced ability of charge transport, which is consistent with its good photoresponse. Therefore, all of the PEC measurements clearly indicate that Cu2O/G/FNA composite exhibits much more efficient generation, separation and transport of photogenerated charge carriers than 1C, 2C and 3C non-Z-scheme system, thus expecting the improved photocatalytic performance. To further study the effect of 3C Z-scheme system on the photocatalytic activity, the degradation of MB solution under visible-light irradiation was evaluated (Fig. 5(a)). After 30-min adsorption/desorption equilibrium in the dark, Z-scheme Cu2O/G/FNA sample shows
3. Results and discussions The XRD patterns of as-prepared samples are shown in Fig. 1(a). All the samples confirm the crystalline hematite phase (JCPDS No. 33–0664) and pure iron phase (JCPDS No. 65–4899) [7] whereas the peaks at 36.5°, and 42.3° are observed ascribed to the (111) and (200) crystal planes of Cu2O (JCPDS No. 34–1354) for Cu2O/FNA, Cu2O/G/ FNA, and G/Cu2O/FNA samples [13]. These strong and sharp peaks shown in each XRD pattern demonstrate that the obtained samples are well crystallized. However, the absence of the characteristic peak of G is possibly related to the thinness of the G layer [1]. To further evidence the existence of G, Raman spectra of GO and Cu2O/G/FNA samples are characterized, shown in Fig. 1(b). The peaks at ~ 1329 cm−1 (D band) and ~ 1598 cm−1 (G band) belong to the characteristic peaks of graphitic carbon [14]. In addition, the increase of D/G intensity ratio of Cu2O/G/FNA (1.85) compared to that of GO (1.28) also confirms the reduction of GO into G (or reduced graphene oxide (RGO)) coated on the FNA substrate during the electrodeposition process [1,14]. To further determine the chemical composition of Cu2O/G/FNA, the highsolution XPS of Cu 2p over Cu2O/G/FNA is shown in Fig. 1(c). The peaks at binding energies of 932.9 and 952.7 eV is correspond to Cu 2p3/2 and Cu 2p1/2 of Cu2O while those at 934.3 and the satellite peak around 940–945 eV demonstrates the coexistence of a trace amount of CuO in the Cu2O/G/FNA sample owing to the partial oxidation of Cu+ during the preparation process [7,9]. Fig. 1(d) and Fig. S2 shows FE-SEM images of all samples. The FNA have a compact array of vertically aligned nanotubes with an average inner diameter of ~ 40 nm (Fig. S2(a)). After the cyclic voltammetric deposition and reduction, multilayers graphene sheets forms in G/FNA (Fig. S2(b)). Moreover, for Cu2O/FNA, close-packed sphere-shaped Cu2O nanoparticles with the diameters of ~ 50 nm are built up on the top surface of FNA (Fig. S2(c)), and the same 2
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Fig. 1. (a) XRD patterns of all samples and (b) Raman spectra of GO and Cu2O/G/FNA (c) high resolution XPS of Cu 2p for Cu2O/G/FNA; (d) FE-SEM images of Cu2O/G/FNA and the inset is the cross-sectional image.
Fig. 2. (a) UV–vis DRS and (b) Tauc plots of different samples.
and holes have a direct influence on photocatalytic activity [1]. Consequently, the radical trapping experiments further confirm that •O2− radical is the primary active specie over Cu2O/G/FNA in the photodegradation of MB while holes over G/Cu2O/FNA (Fig. 5(c) and (d)). Meanwhile, in order to testify the important role of G sheet as the electron mediator, Cu2O/FNA was used for the radical trapping experiment under the identical condition with the result shown in Fig. S4. From the result, it is seen that holes shows a remarkable dependence on the decomposition of MB, which can be attributed to direct Type II charge transfer between Cu2O and FNA in line with the conclusion reported in the publication [9].
the apparently higher photodegradation rate (~ 86.2%) in 60 min than that of other samples, which is responsible for the stimulative effect of Z-scheme charge transport on the enhanced photoresponse. Moreover, Cu2O/G/FNA clearly shows a higher photostability than Cu2O/FNA (Fig. 5(b)). This may be because of the quenching of oxidative holes on Cu2O by the electrons migrating from FNA, resulting in keeping Cu2O from oxidation [1,7,12], which is also proved by the stable XRD pattern, XPS and Raman spectra (Fig. S3). These results demonstrate that 3C Z-scheme not only enhanced the photocatalytic activity but also greatly improved the photostability. On the other hand, the type and amount of the active species originated from photogenerated electron 3
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average of the absolute electronegativity of the constituent atoms (χ values of α–Fe2O3 and Cu2O are 5.88 eV and 5.32 eV, respectively [9]; and Eg is the band gap energy of the semiconductor. For the sake of acquiring the Eg value of pure Cu2O, Cu2O/Fe sample was fabricated via the identical electrochemical method on the Fe substrate, whose experiment detail is narrated in the Supplementary information. From the UV–vis DRS and Tauc plot of Cu2O/Fe (Fig. S5), the calculated Eg is 2.15 eV corresponding to the reported result [7,15–17]. The positions of the conduction and valence bands of α–Fe2O3 and Cu2O on the absolute vacuum scale calculated by Eqs. (2) and (3) are listed in Table 1. According to the above results, the possible mechanism of photodegradation of MB over Cu2O/G/FNA was illustrated in Fig. 6(a). Whether the photogenerated charge carriers could trigger the redox reactions strongly depends on the band edge positions and the redox potential of reaction species. Generally, the redox potential value of reactants is given vs. normal hydrogen electrode (NHE) while the band edge values is predicted by Mulliken electronegativity theory vs. the absolute vacuum scale. In order to facilitate comparison with them, we recalculated both with respective to NHE according to the relationship between the vacuum energy (Eabs) and the normal electrode potential (Eө) [7,18]:
Table 1 The band gap energies of all samples; the electronegativity, conduction band edge and valence band edge potentials of FNA and Cu2O. Sample
FNA Cu2O (Cu2O/Fe)* G/FNA Cu2O/FNA Cu2O/G/FNA G/Cu2O/FNA *
χ
Eg
ECB
eV
eV
eV vs. vacuum
eV vs. NHE pH = 7
eV vs. vacuum
eV vs. NHE pH = 7
5.88 5.32
2.08 2.15 2.09 2.14 2.12 2.11
− 4.84 − 4.245
− 0.01 − 0.61
− 6.92 − 6.395
2.07 1.54
EVB
The Eg value of Cu2O was calculated by the UV–vis DRS of Cu2O/Fe sample.
Eabs = − E θ − 4.44
(4)
In addition, because the pH value of our photocatalytic system was adjusted to 7, the calculated band edge values and the redox potential of reactants are both converted in accordance with Nernst equation [19,20]:
E = E θ −0.059pH
The corresponding EVB and ECB results are also listed in Table 1. On the basis of Type II heterojunction charge transport, the photoelectrons on the CB of Cu2O (− 0.61 eV vs. NHE, pH = 7) will transfer to the CB of FNA (− 0.01 eV vs. NHE, pH = 7), which is not negative enough to effectively reduce O2 to form •O2− (− 0.46 V vs. NHE, pH = 7) [11]. Considering the •O2− is the main active species and the obviously improved photoactivity of 3C Z-scheme composite, this is not in agreement with the obtained experimental results. However, if based on Z-scheme charge transfer, electrons can transfer from the CB of FNA to the VB of Cu2O via G-sheet layer and quench with holes successively. Thus, in the light of redox potential of active species and energy band levels of 3C composite material, strong reductive electrons on the CB of Cu2O can be retained and react with O2 into •O2− while strong oxidative holes accumulating on the VB of FNA can oxidize water to generate •OH radicals (2.04 V vs. NHE, pH = 7), thus decomposing dye molecules [21]. This would be beneficial to the promotion of photocatalytic activity. On the contrary, for 3C non-Zscheme composite, graphene on the surface of Cu2O/FNA cannot used
Fig. 3. PL spectra of FNA, Cu2O/G/FNA and G/Cu2O/FNA.
In order to explain the photogenerated charge carriers transport mechanism on the 3 C composite, the band edge positions of FNA and Cu2O were estimated. The conduction band and valence band potentials of a semiconductor can be predicted by Mulliken electronegativity theory [9]:
EVB = − χ −0.5Eg
(2)
ECB = Eg − EVB
(3)
(5)
where EVB and ECB is the VB edge and CB edge potential, respectively; χ is the electronegativity of the semiconductor, which is the geometric
Fig. 4. (a) photocurrent curves for different samples under visible-light irradiation; (b) EIS Nyquist plots of FNA, Cu2O/G/FNA and G/Cu2O/FNA at open-circuit voltage under visible-light irradiation.
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Fig. 5. (a) Photodegradation of MB for all samples and control experiments under visible-light irradiation; (b) photostability over Cu2O/FNA (I) and Cu2O/G/FNA (II) under visible-light irradiation; plots of radicals trapping over Cu2O/G/FNA (c) and G/Cu2O/FNA (d);.
Fig. 6. Possible mechanism illustrations for photodegradation of MB over Cu2O/G/FNA in Z-scheme (a) and G/Cu2O/FNA in non-Z-scheme (b).
4. Conclusion
as an electron mediator to induce Z-scheme charge transport. The transport and accumulation of electrons and holes between different energy band levels causes the lower reduction or oxidation potentials of electrons and holes, which fail to trigger the relevant radical generation reactions (Fig. 6(b)). Therefore, the photoactivity of G/Cu2O/FNA was restrained largely. From the results of radical trapping experiments over G/Cu2O/FNA, it is proved that holes are the main active species, also supporting the non-Z-scheme mechanism we proposed. Hence, Z-scheme charge transport could not only effectively achieve the spatial separation of the CB electrons of Cu2O with a strong reduction potential and the VB holes of FNA with a strong oxidation potential but also inhibit the photocorrosion of Cu2O by quenching the strong oxidative holes on the VB of Cu2O, which explains the high photoactivity and photostability of Z-scheme Cu2O/G/FNA composite.
A novel Z-scheme Cu2O/G/FNA composite has been successfully produced via facile and successively electrochemical processes. This composite exhibited superior visible-light absorption and efficient spatial separation of charge carriers due to Z-scheme charge transport. Through the PEC measurements and photodegradation of MB under visible-light irradiation this 3C Z-scheme composite exhibited a higher photoactivity and photostability than other samples. The Z-scheme mechanism of photodegradation of MB was also confirmed by radical trapping experiments, revealing that •O2− radicals is the prime active species over Cu2O/G/FNA. In addition, G-sheet layer as an efficient electron mediator plays a key role in the construction of Z-scheme 3C composite photocatalyst.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Construction of novel Z-scheme Cu2O/graphene/α-Fe2O3 nanotube arrays composite for enhanced photocatalytic activity ⁎
Fei Li , Bo Dong School of Mechanical Engineering, Jiangxi University of Technology, Nanchang 330098, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Z-scheme Cu2O/graphene/α-Fe2O3 Composites Photocatalytic degradation Methylene blue
Novel Z-scheme Cu2O/graphene/α-Fe2O3 nanotube arrays (Cu2O/G/FNA) composite was synthesized by facile electrochemical processes for the first time. Compared to single- (1C) or two-component (2C) system, threecomponent (3C) Z-scheme composite material exhibited the efficient photogenerated electron-hole separation as well as the sufficient redox potential via directly quenching between the weak oxidative holes and reductive electrons after Z-scheme charge transfer. The photoelectrochemical measurements and photocatalytic degradation of methylene blue by Cu2O/G/FNA under visible-light irradiation demonstrated the enhanced photoactivity and photostability compared to 1C, 2C, and 3C non-Z-scheme system. The Z-scheme photocatalytic mechanism was confirmed based on the relative characterizations and radical trapping experiments over Cu2O/G/FNA and G/Cu2O/FNA composites, also indicating that •O2− radicals was the prime active species over Cu2O/G/FNA. In addition, G-sheet layer as an efficient electron mediator plays a key role in the construction of Z-scheme 3C composite photocatalyst.
1. Introduction Photocatalysis as an advanced oxidative process using semiconductors such as TiO2 has attracted much attention for wastewater treatment [1–5]. However, due to the large bandgap of TiO2 (~ 3.2 eV) its practical photocatalytic applications are limited at solar light illumination [1]. Comparison with TiO2, hematite (α-Fe2O3, ntype semiconductor) is believed to be a promising candidate due to its suitable bandgap of ~ 2.0 eV for efficient visible-light harvesting, low cost and environmentally friendly characteristics [6]. However, the photocatalytic performance of α-Fe2O3 is still hinted by bulk and surface electron-hole recombination [7]. In order to inhibit this electron-hole recombination, one-dimension α-Fe2O3 nanotube arrays (FNA) was employed as its large specific surface area and short holes transport route [8]. Although this, the problem of surface recombination of photogenerated electron-hole pairs remains. To thoroughly overcome this drawback, two-component (2C) semiconductors heterojunction composite has been extensively studied. Among these alternative semiconductor materials, Cu2O as an attractive p-type semiconductor (2.0–2.2 eV) can be conjugated with hematite to enhance the separation of photogenerated carriers because of the Type II charge transport between their matched energy band positions [7,9,10]. Nevertheless, this traditional charge transfer route between heterojunctions leads to the weak redox potential of photogenerated electrons
⁎
and holes [11]. Recently, Z-scheme photocatalyst composite has been developed which not only retains the efficient photogenerated charge separation but achieves its excellent redox ability [1,11,12]. Although Z-scheme α-Fe2O3/Cu2O heterostructure has been reported so far [7], to our knowledge, the research on Z-scheme FNA-based heterojunction materials with enhanced photocatalytic performance is still limited. On the basis above, we adopted graphene (G) as a good electron mediator to successfully fabricate three-component (3C) Z-scheme Cu2O/G/FNA composite for the first time. Compared to single-component (1C) and 2C systems, it showed the obviously improved photocatalytic activity and photostability for the degradation of methylene blue (MB) under visible-light irradiation (λ ≥ 400 nm). The Z-scheme charge transport route and photocatalytic mechanism were also confirmed on the basis of the relative characterizations, photoelectrochemical (PEC) measurements and radical trapping experiments over pure FNA, 1C, 2C and 3C FNA-based composite materials. 2. Experiments and characterizations The Z-scheme Cu2O/G/FNA composite was prepared by facile and successive electrochemical processes. The detail synthesis procedures are available in the Supplementary information. The morphologies of the samples were observed on JEOL7600F field emission electron microscope (FE-SEM). X-ray diffraction (XRD)
Corresponding author. E-mail address: [email protected] (F. Li).
http://dx.doi.org/10.1016/j.ceramint.2017.08.021 Received 20 May 2017; Received in revised form 18 July 2017; Accepted 2 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, F., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.021
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phenomenon has also been observed in the Cu2O/G/FNA sample (Fig. 1(d)), which reveals the introduction of G has no influence on the morphology of Cu2O nanoparticles and FNA substrate. In addition, for G/Cu2O/FNA, Fig. S2(d) shows that Cu2O nanoparticles are only located on the mouth of FNA, and covered by a few of G layers, which is in accord with non-Z-scheme architecture illustrated in the Fig. S1. Thus, from the results of FE-SEM, it is obviously demonstrated that the 3C layer-structure composites have been successfully fabricated, which is also verified on the basis of XRD, Raman spectra and XPS. The UV–vis DRS show that the main absorption range of bare FNA extends into the visible region (Fig. 2(a)). After coupling with G or Cu2O, the absorption intensity of both the G/FNA and Cu2O/FNA samples decreases to some extent, resulting from the opacity of G [14] and the wider band gap of Cu2O relative to α-Fe2O3 [7]. Although this, Cu2O/G/FNA sample shows the strong visible-light absorption, indicating the practical feasibility as efficient visible-light photocatalyst. However, the 3 C composite G/Cu2O/FNA with the identical components exhibits the apparently poor visible-light absorption due to the block effect of the G layers on the top surface. According to Tauc equation, the band gap energies were calculated using the following Equation [10]:
pattern was performed on D8 Advance X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectra (XPS) and Raman spectra were recorded on an XSAM-800 X-ray Photoelectron Spectroscopy and Spex 403 Raman spectrometer. Photoluminescence (PL) and UV–vis diffuse reflectance spectra (DRS) were taken at room temperature with a fluorospectrophotometer with an excitation at 350 nm light, and a Cary 5000 spectrometer. Photoelectrochemical measurements were performed on an electrochemical workstation in a three-electrode 0.5 M Na2SO4 solution system using the as-prepared sample foils as the working electrodes (cut into 1 cm × 5 cm in size) with a light area of 0.5 cm × 0.5 cm = 0.25 cm2, a Pt plate as the counter electrode, and a Ag/AgCl (saturated in KCl) as the reference electrode. The light source was a xenon lamp irradiation through a filter (λ ≥ 400 nm) with an intensity of 100 mW/cm2. The transient photocurrent curves were measured at + 0.6 V vs. Ag/AgCl under visible-light illumination with on/off light cycles of 50 s intervals. Electrochemical impedance spectroscopy (EIS) tests were conducted at open-circuit voltage under the identical irradiation with frequency in the range of 1–106 Hz and an amplitude of 5 mV. In the photodegradation of MB, the obtained samples (2 × 2 cm2) were placed into a 50 mL MB solution (20 mg/L, pH = 7) under continuous stirring. After 30 min adsorption-desorption equilibrium, the degradation efficiency of MB under visible-light irradiation (λ ≥ 400 nm) were recorded in 60 min by the absorption intensity of the solution at 664 nm in the measured UV–vis spectra. The photocatalytic degradation efficiency of MB were evaluated by the value of C/ C0, where C0 is the absorbance of original MB solution, C is the absorbance of the dye solution after light irradiation. Radical trapping experiments over Cu2O/FNA, Cu2O/G/FNA and G/Cu2O/FNA sample under identical conditions were separately performed by adding 1 mM ammonium oxalate (AO), 1 mM p-benzoquinone (BQ) or 1 mM tert-butyl alcohol (TBA) into the MB solution before the photocatalytic tests to trap holes, •O2− or •OH, respectively.
αhν = A(hν − Eg )η
(1)
where α, h, ν, A and Eg are η absorption coefficient, plank's constant, frequency of light, proportionality constant and band gap energy, respectively. η is a variable which depends on the nature of optical transition (for direct band transition η = 0.5 and for indirect band transition η = 2). Hence, Tauc plots of all samples were shown in the Fig. 2(b) and the calculated Eg was listed in the Table 1. The calculated Eg value of FNA is 2.08 eV, agreeing well with the reported values of α– Fe2O3 [6]. The Eg values of the composites are found to be almost unchanged. This indicates the introduction of G and Cu2O have no remarkable effect on the band gap energy of composites. What's more, Fig. 3 shows the lowest PL intensity of the Cu2O/G/ FNA composite, suggesting the lowest recombination rate of electronhole pairs and longest lifetime of photogenerated carriers. Especially, compared with G/Cu2O/FNA sample, the remarkably lower PL intensity of Cu2O/G/FNA has been exhibited, testifying that Z-scheme charge transport would be in favor of the effective separation of electrons and holes [7,12]. To detailedly characterize the photoresponse of different samples, photoelectrochemical (PEC) measurements were performed in a threeelectrode system. Transient photocurrent responses are recorded over several light on-off illumination cycles to investigate the separation of photogenerated electron-hole pairs, as shown in Fig. 4(a). From the curves of photocurrent over all samples, it is showed that the PEC response of Cu2O/G/FNA is remarkably better than that of other samples, indicating the increasing charge carriers generation and enhanced separation of electron-hole pairs in 3C Z-scheme system in comparison with 1C, 2C and 3C non-Z-scheme system. Additionally, EIS measurement can manifest the conductivity property of the materials under the irradiation. As can be seen in Fig. 4(b), the diameter of semicircle at high frequency in a Nyquist plot, equal to the charge transfer resistance, over Cu2O/G/FNA sample is much smaller than those of pure FNA and G/Cu2O/FNA, implying its enhanced ability of charge transport, which is consistent with its good photoresponse. Therefore, all of the PEC measurements clearly indicate that Cu2O/G/FNA composite exhibits much more efficient generation, separation and transport of photogenerated charge carriers than 1C, 2C and 3C non-Z-scheme system, thus expecting the improved photocatalytic performance. To further study the effect of 3C Z-scheme system on the photocatalytic activity, the degradation of MB solution under visible-light irradiation was evaluated (Fig. 5(a)). After 30-min adsorption/desorption equilibrium in the dark, Z-scheme Cu2O/G/FNA sample shows
3. Results and discussions The XRD patterns of as-prepared samples are shown in Fig. 1(a). All the samples confirm the crystalline hematite phase (JCPDS No. 33–0664) and pure iron phase (JCPDS No. 65–4899) [7] whereas the peaks at 36.5°, and 42.3° are observed ascribed to the (111) and (200) crystal planes of Cu2O (JCPDS No. 34–1354) for Cu2O/FNA, Cu2O/G/ FNA, and G/Cu2O/FNA samples [13]. These strong and sharp peaks shown in each XRD pattern demonstrate that the obtained samples are well crystallized. However, the absence of the characteristic peak of G is possibly related to the thinness of the G layer [1]. To further evidence the existence of G, Raman spectra of GO and Cu2O/G/FNA samples are characterized, shown in Fig. 1(b). The peaks at ~ 1329 cm−1 (D band) and ~ 1598 cm−1 (G band) belong to the characteristic peaks of graphitic carbon [14]. In addition, the increase of D/G intensity ratio of Cu2O/G/FNA (1.85) compared to that of GO (1.28) also confirms the reduction of GO into G (or reduced graphene oxide (RGO)) coated on the FNA substrate during the electrodeposition process [1,14]. To further determine the chemical composition of Cu2O/G/FNA, the highsolution XPS of Cu 2p over Cu2O/G/FNA is shown in Fig. 1(c). The peaks at binding energies of 932.9 and 952.7 eV is correspond to Cu 2p3/2 and Cu 2p1/2 of Cu2O while those at 934.3 and the satellite peak around 940–945 eV demonstrates the coexistence of a trace amount of CuO in the Cu2O/G/FNA sample owing to the partial oxidation of Cu+ during the preparation process [7,9]. Fig. 1(d) and Fig. S2 shows FE-SEM images of all samples. The FNA have a compact array of vertically aligned nanotubes with an average inner diameter of ~ 40 nm (Fig. S2(a)). After the cyclic voltammetric deposition and reduction, multilayers graphene sheets forms in G/FNA (Fig. S2(b)). Moreover, for Cu2O/FNA, close-packed sphere-shaped Cu2O nanoparticles with the diameters of ~ 50 nm are built up on the top surface of FNA (Fig. S2(c)), and the same 2
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Fig. 1. (a) XRD patterns of all samples and (b) Raman spectra of GO and Cu2O/G/FNA (c) high resolution XPS of Cu 2p for Cu2O/G/FNA; (d) FE-SEM images of Cu2O/G/FNA and the inset is the cross-sectional image.
Fig. 2. (a) UV–vis DRS and (b) Tauc plots of different samples.
and holes have a direct influence on photocatalytic activity [1]. Consequently, the radical trapping experiments further confirm that •O2− radical is the primary active specie over Cu2O/G/FNA in the photodegradation of MB while holes over G/Cu2O/FNA (Fig. 5(c) and (d)). Meanwhile, in order to testify the important role of G sheet as the electron mediator, Cu2O/FNA was used for the radical trapping experiment under the identical condition with the result shown in Fig. S4. From the result, it is seen that holes shows a remarkable dependence on the decomposition of MB, which can be attributed to direct Type II charge transfer between Cu2O and FNA in line with the conclusion reported in the publication [9].
the apparently higher photodegradation rate (~ 86.2%) in 60 min than that of other samples, which is responsible for the stimulative effect of Z-scheme charge transport on the enhanced photoresponse. Moreover, Cu2O/G/FNA clearly shows a higher photostability than Cu2O/FNA (Fig. 5(b)). This may be because of the quenching of oxidative holes on Cu2O by the electrons migrating from FNA, resulting in keeping Cu2O from oxidation [1,7,12], which is also proved by the stable XRD pattern, XPS and Raman spectra (Fig. S3). These results demonstrate that 3C Z-scheme not only enhanced the photocatalytic activity but also greatly improved the photostability. On the other hand, the type and amount of the active species originated from photogenerated electron 3
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average of the absolute electronegativity of the constituent atoms (χ values of α–Fe2O3 and Cu2O are 5.88 eV and 5.32 eV, respectively [9]; and Eg is the band gap energy of the semiconductor. For the sake of acquiring the Eg value of pure Cu2O, Cu2O/Fe sample was fabricated via the identical electrochemical method on the Fe substrate, whose experiment detail is narrated in the Supplementary information. From the UV–vis DRS and Tauc plot of Cu2O/Fe (Fig. S5), the calculated Eg is 2.15 eV corresponding to the reported result [7,15–17]. The positions of the conduction and valence bands of α–Fe2O3 and Cu2O on the absolute vacuum scale calculated by Eqs. (2) and (3) are listed in Table 1. According to the above results, the possible mechanism of photodegradation of MB over Cu2O/G/FNA was illustrated in Fig. 6(a). Whether the photogenerated charge carriers could trigger the redox reactions strongly depends on the band edge positions and the redox potential of reaction species. Generally, the redox potential value of reactants is given vs. normal hydrogen electrode (NHE) while the band edge values is predicted by Mulliken electronegativity theory vs. the absolute vacuum scale. In order to facilitate comparison with them, we recalculated both with respective to NHE according to the relationship between the vacuum energy (Eabs) and the normal electrode potential (Eө) [7,18]:
Table 1 The band gap energies of all samples; the electronegativity, conduction band edge and valence band edge potentials of FNA and Cu2O. Sample
FNA Cu2O (Cu2O/Fe)* G/FNA Cu2O/FNA Cu2O/G/FNA G/Cu2O/FNA *
χ
Eg
ECB
eV
eV
eV vs. vacuum
eV vs. NHE pH = 7
eV vs. vacuum
eV vs. NHE pH = 7
5.88 5.32
2.08 2.15 2.09 2.14 2.12 2.11
− 4.84 − 4.245
− 0.01 − 0.61
− 6.92 − 6.395
2.07 1.54
EVB
The Eg value of Cu2O was calculated by the UV–vis DRS of Cu2O/Fe sample.
Eabs = − E θ − 4.44
(4)
In addition, because the pH value of our photocatalytic system was adjusted to 7, the calculated band edge values and the redox potential of reactants are both converted in accordance with Nernst equation [19,20]:
E = E θ −0.059pH
The corresponding EVB and ECB results are also listed in Table 1. On the basis of Type II heterojunction charge transport, the photoelectrons on the CB of Cu2O (− 0.61 eV vs. NHE, pH = 7) will transfer to the CB of FNA (− 0.01 eV vs. NHE, pH = 7), which is not negative enough to effectively reduce O2 to form •O2− (− 0.46 V vs. NHE, pH = 7) [11]. Considering the •O2− is the main active species and the obviously improved photoactivity of 3C Z-scheme composite, this is not in agreement with the obtained experimental results. However, if based on Z-scheme charge transfer, electrons can transfer from the CB of FNA to the VB of Cu2O via G-sheet layer and quench with holes successively. Thus, in the light of redox potential of active species and energy band levels of 3C composite material, strong reductive electrons on the CB of Cu2O can be retained and react with O2 into •O2− while strong oxidative holes accumulating on the VB of FNA can oxidize water to generate •OH radicals (2.04 V vs. NHE, pH = 7), thus decomposing dye molecules [21]. This would be beneficial to the promotion of photocatalytic activity. On the contrary, for 3C non-Zscheme composite, graphene on the surface of Cu2O/FNA cannot used
Fig. 3. PL spectra of FNA, Cu2O/G/FNA and G/Cu2O/FNA.
In order to explain the photogenerated charge carriers transport mechanism on the 3 C composite, the band edge positions of FNA and Cu2O were estimated. The conduction band and valence band potentials of a semiconductor can be predicted by Mulliken electronegativity theory [9]:
EVB = − χ −0.5Eg
(2)
ECB = Eg − EVB
(3)
(5)
where EVB and ECB is the VB edge and CB edge potential, respectively; χ is the electronegativity of the semiconductor, which is the geometric
Fig. 4. (a) photocurrent curves for different samples under visible-light irradiation; (b) EIS Nyquist plots of FNA, Cu2O/G/FNA and G/Cu2O/FNA at open-circuit voltage under visible-light irradiation.
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Fig. 5. (a) Photodegradation of MB for all samples and control experiments under visible-light irradiation; (b) photostability over Cu2O/FNA (I) and Cu2O/G/FNA (II) under visible-light irradiation; plots of radicals trapping over Cu2O/G/FNA (c) and G/Cu2O/FNA (d);.
Fig. 6. Possible mechanism illustrations for photodegradation of MB over Cu2O/G/FNA in Z-scheme (a) and G/Cu2O/FNA in non-Z-scheme (b).
4. Conclusion
as an electron mediator to induce Z-scheme charge transport. The transport and accumulation of electrons and holes between different energy band levels causes the lower reduction or oxidation potentials of electrons and holes, which fail to trigger the relevant radical generation reactions (Fig. 6(b)). Therefore, the photoactivity of G/Cu2O/FNA was restrained largely. From the results of radical trapping experiments over G/Cu2O/FNA, it is proved that holes are the main active species, also supporting the non-Z-scheme mechanism we proposed. Hence, Z-scheme charge transport could not only effectively achieve the spatial separation of the CB electrons of Cu2O with a strong reduction potential and the VB holes of FNA with a strong oxidation potential but also inhibit the photocorrosion of Cu2O by quenching the strong oxidative holes on the VB of Cu2O, which explains the high photoactivity and photostability of Z-scheme Cu2O/G/FNA composite.
A novel Z-scheme Cu2O/G/FNA composite has been successfully produced via facile and successively electrochemical processes. This composite exhibited superior visible-light absorption and efficient spatial separation of charge carriers due to Z-scheme charge transport. Through the PEC measurements and photodegradation of MB under visible-light irradiation this 3C Z-scheme composite exhibited a higher photoactivity and photostability than other samples. The Z-scheme mechanism of photodegradation of MB was also confirmed by radical trapping experiments, revealing that •O2− radicals is the prime active species over Cu2O/G/FNA. In addition, G-sheet layer as an efficient electron mediator plays a key role in the construction of Z-scheme 3C composite photocatalyst.
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